Polymers and copolymers of hydroxy acids, generally known as poly(hydroxy acid)s, slowly hydrolyze and biodegrade to environmentally benign products. These materials are well behaved thermoplastics with appealing aesthetic qualities. Consequently, high molecular weight poly(hydroxy acid)s, particularly polymers and copolymers of lactic acid and glycolic acid, are potential replacements for poly(styrene) and other non-biodegradable polymers in numerous applications, especially packaging.
It is desirable for packaging material to possess additional beneficial physical properties including optical clarity, tensile strength, elasticity, resistance to puncture and, optionally, to be shrinkable. Shrinkable films are useful for "shrink-wrap" packaging. Shrink-wrapping is the process by which a film structure placed around an object contracts to provide a tight, protective, self-sealing container, upon application of convective or radiant heat. In order to make poly(hydroxy acid) films shrinkable, they must be oriented by mechanical stretching procedures.
Many high molecular weight poly(hydroxy acid)s of commercial significance contain large proportions of specific asymmetric carbon atoms and may be, therefore, appreciably crystalline and brittle. Such polymers are less able to withstand demanding film-forming and orientation processes without excessive film breakage causing low yield and productivity. Films produced from these polymers can become brittle on aging.
Poly(hydroxy acid) film can be made more pliable by incorporation of a plasticizer, especially large amounts of monomeric and low molecular weight oligomeric 2-hydroxyacid species. Sinclair, U.S. Pat. No. 5,180,765, describes poly(lactic acid) plasticized with monomers and low molecular weight oligomers of lactic acid. However, such plasticized poly(hydroxy acid) compositions can be difficult to extrude into films because they stick to the film-making equipment, produce films of non-uniform thickness and the plasticizer often fouls the equipment. Even when successfully processed to film form, significantly plasticized poly(hydroxy acid) compositions have glass transition temperatures that are near ambient temperatures and may not be stable under storage conditions found in warm climates. In addition, they may not have appropriate rate of degradation to maintain structural integrity during the intended life of the package. A need exists for significantly degradable compositions that may be processed economically, and with high yield, into optically clear, robust films for commercial packaging and other applications.